This invention is directed to the production of glass compositions suitable for use as faceplates for projection cathode ray tubes; i.e., television picture tubes used to project images on a large screen. Faceplates for use in projection cathode ray tubes have been commercially fabricated for a number of years from glasses having compositions encompassed within U.S. Pat. No. 4,277,286. The preferred glasses disclosed in that patent consisted essentially, in weight percent, of:
______________________________________ SiO.sub.2 48-53 K.sub.2 O 5-6 SrO 3-7 Al.sub.2 O.sub.3 1-2 CeO.sub.2 0.4-0.8 BaO 14-16 ZnO 7-11 TiO.sub.2 0.5-1.0 CaO + SrO + BaO 18-24 Li.sub.2 O 1-3 ZrO.sub.2 2-7 Li.sub.2 O + Na.sub.2 O + K.sub.2 O 11-16 Na.sub.2 O 5-7 CaO 0-3 ______________________________________
Those glasses exhibited exceptional absorptions of X-radiation (linear X-ray absorption coefficients at 0.6.ANG. of at least 35 cm.sup.-1) and very little browning from subjection to high velocity electron impingement. Those glasses do, however, demonstrate one undesirable feature; viz., faceplates prepared therefrom appear to be slightly yellow to the eye. This yellow coloration, caused by absorption in the blue end of the visible radiation spectrum, renders it necessary for the electron gun utilized in developing the blue color in the projection television system to be run "harder," i.e., to be operated at a higher voltage, than the electron guns used for generating the red and green colors. Hence, the yellow color is not only undesirable from an aesthetic standpoint, but also from a tube operation point of view.
Therefore, the principal objective of the present invention was to devise glass compositions having the exceptional absorption of X-radiation and resistance to browning resulting from the impingement of high velocity electrons demonstrated by the glasses of U.S. Pat. No. 4,277,286, but which would exhibit essentially no yellow color to the eye.